1. Field of the Invention
The present invention generally concerns (i) microstructures including optical microlenses and electrical bump bonds; (ii) liquids (and especially liquid monomers that may be cured to form polymers) and the use of such liquids in the fabrication of microstructures; and (iii) the use of hydrophobic and hydrophilic effects in the fabrication of microstructures, including optical microlenses, electrical bump bonds, and electrical bonds between mutually perpendicular chips.
The present invention particularly concerns the precision fabrication of, inter alia, (i) high performance transparent polymer optical microlenses and microlens arrays, including as may be either arrayed or aligned, including self-aligned, to the ends of optical fibers, and, separately by an analogous process, (ii) electrically conductive polymer bump bonds, and conductive bump bonds self-aligned to bump pads, and, separately by an analogous process, (iii) bonds between mutually perpendicular substrates.
In all areas (i)-(iii) the present invention still more particularly concerns microlens or bump bond or substrate(s) bond fabrication by (1) transfer of a liquid polymer precursor onto hydrophilic domains of a substrate of patterned wettability followed by (2) curing of the polymer; said (1) transfer of liquid being realized either by (1a) condensing the liquid onto these hydrophilic domains or (1b) withdrawing substrates of patterned wettability from a liquid solution at controlled speed and said (2) curing being realized by (2a) heat, (2b) chemical reaction or (2c) ultraviolet light.
In area (i) the present invention still more particularly concerns aligning microlenses to optical components, and the direct fabrication of microlenses on optical components.
Also in the area (i) the present invention still more particularly concerns the low cost fabrication of microlenses that are self-aligned to optical fibers and/or low-wavelength (<500 nm) single-mode light output devices, including microlenses as may be fabricated both (i) directly upon these fibers or components or (ii) in a precisely spaced and oriented relationship thereto.
In the area (ii), the electrically conductive polymer bump bonds—the fabrication of which is an area of the present invention—may still more particularly be (i) arrayed and/or (ii) self-aligned, by the use of hydrophobic and hydrophilic effects.
Likewise in the area (iii), the electrically conductive bonds between perpendicular substrates—the fabrication of which is another area of the present invention—may still more particularly be (i) arrayed and/or (ii) self-aligned, to either or to both substrates by the use of hydrophobic and hydrophilic effects.
2. Description of the Prior Art
2.1 Microlenses
In today's world (circa 2001) of information processing, the role of arrayed optics is becoming more and more important as the need for parallelism and density increases in each of display, communication, and storage applications. The trend towards highly parallel compact optical systems has in particular lead to a growing need for high-performance, low f-number (f#), microlens arrays.
Refractive microlenses have been utilized in hybrid optical interconnect strategies. See, for example, M. R. Taghizadeh “Micro-optical fabrication technologies for optical interconnection applications”, in Diffractive Optics and Micro-Optics, OSA Technical Digest, 260 (2000); M. W. Haney “Micro- vs. macro-optics in free-space optical interconnects” in Diffractive Optics and Micro-Optics, OSA Technical Digest, 266 (2000); S. Eitel, S. J. Fancey, H. P. Gauggel, K. H. Gulden, W. Bachtold, M. R. Taghizadeh, “Highly uniform vertical-cavity surface-emitting lasers integrated with microlens arrays”, IEEE Photonics Technology Letters 12, IEEE,. 459-61 (2000); and G. Sharp, L. E. Schmutz, “Microlens arrays meet any challenge”, Lasers & Optronics Lasers Optronics (USA) 16, 21-3 (1997).
Refractive microlenses have also been used in switching networks. See, for example, M. C. Wu, L. Y. Lin, S. S. Lee, C. R. King, “Free-space integrated optics realized by surface-micromachining”, International Journal of High Speed Electronics and Systems 8, World Scientific, 283-97 (1997); and M. F. Chang, M. C. Wu, J. J. Yao, M. E. Motamedi, “Surface micromachined devices for microwave and photonic applications”, Proceedings of the SPIE—The International Society for Optical Engineering, (Optoelectronic Materials and Devices) 3419, M. Osinski, Y. Su, chairs/editors, 214-26 (1998).
Refractive microlenses have further been used in spectrophotometry. See, for example, S. Traut, H. P. Herzig, “Holographically recorded gratings on microlenses for a miniaturized spectrometer array”, Optical Engineering 39, 290-8 (2000).
Refractive microlenses have still further been used in confocal microscopy. See, for example, M. Eisner, N. Lindlein, J. Schwider, “Confocal microscopy with a refractive microlens-pinhole array”, Optics Letters 23, 748-9 (1998).
Refractive microlenses have yet still further been used in sensors. See, for example, P. Nussbaum, R. Volkel, H. P. Herzig, M. Eisner, S. Haselbeck, “Design, fabrication and testing of microlens arrays for sensors and Microsystems”, Pure and Applied Optics 6, 617-36 (1997).
Refractive microlenses have yet still further been used in focal plane arrays. See, for example, M. E. Motamedi, W. E. Tennant, H. O. Sankur, R. Melendes, N. S. Gluck, S. Park, J. M. Arias, J. Bajaj, J. G. Pasko, W. V. McLevige, M. Zandian, R. L. Hall, P. D. Richardson, “Micro-optic integration with focal plane arrays”, Optical Engineering 36, 1374-81 (1997).
Finally, refractive microlenses have been used in photolithography. See, for example, P. Nussbaum, R. Volkel, H. P. Herzig, M. Eisner, S. Haselbeck, “Design, fabrication and testing of microlens arrays for sensors and Microsystems”, Pure and Applied Optics 6, 617-36 (1997).
In general, increasing applications for micro-optical elements present increasing applications for microlenses, and microlens arrays. To address this expanding need for microlenses, fabrication technologies would desirably be identified that will permit precision microlens arrays to be constructed at low cost. In addition, these microlens arrays must be reliably uniform and reproducible so that they can be incorporated seamlessly into existing optical architectures and systems.
2.1.1 Fabrication of Arrays of Microlenses by Other Than Hydrophobic Processes
At present, there are several methods used to form arrays of refractive microlenses. The most viable of these techniques include: (1) dispensed droplets, (2) thermal reflow, and (3) photothermal expansion. A brief description of each of these techniques follows:
In the dispensed droplets technique a modified ink-jet printer, or other dispensing mechanism is used to dispense precise volumes of a polymer material into an array of droplets which can serve as microlenses.
The dispensed droplet technique for fabricating microlens arrays does not utilize a lithographic step, and hence suffers from drawbacks in accuracy. Because of limitations in fluid-handling control, microlenses fabricated with this technique have relatively large minimum diameters (typical values today ˜70 um). The pitch of the lenses must also be relatively large, to avoid overlap. Also, the footprints of the lenses thus created are always circular, unless the substrate is pre-patterned, or a special curing process is employed. Another drawback to the technique of dispensed droplets is that it typically requires heating of the dispensed material, making it incompatible with some heat-sensitive materials. In it's defense, the technique requires very little characterization, and few processing steps, making it low-cost. See for example W. R. Cox, D. J. Hayes, T. Chen, and D. W. Ussery, “Fabrication of micro-optics by microjet printing”, SPIE Vol. 2383, pp. 110-115.
In the thermal reflow technique a substrate is coated with a layer of photoresist. The resist is patterned to form an array of resist “islands”. The substrate is then heated until the resist melts and surface tension draws the islands of resist into the shape of the lenses. If desired, this pattern can then be transferred to the underlying substrate with a reactive ion etch (RIE). Gray scale masks can be used to further extend the capabilities of the thermal reflow process. The use of gray level masks allows the fabrication of “photoresist sculptures”, which can, in principle, be used to fabricate almost any desired lens shape with excellent precession.
Thermal reflow techniques have been used to generate extremely uniform arrays of microlenses with arbitrary footprint shapes. The lenses are lithographically defined, and hence the size, shape, and pitch of the lens arrays can be controlled to within 0.1 um. However there are several drawbacks to this technique. First, it requires extensive characterization of the reflow process, including careful control of resist thickness, and exposure times, and the temperature, and heating times at which the reflow process is conducted. Second, since photoresist is opaque at many wavelengths of interest, it is often necessary to transfer the lens pattern to an underlying substrate, using an RIE. This again requires careful characterization. Because of the complications involved in characterizing the reflow process and the subsequent RIE, the technique is somewhat costly. See for example, Kufner, Maria and Stefan, Micro-optics and Lithography, VUBPRESSS, Brussels, 1997, pp 81-118, 183-184. See also Herzig, Hans Peter, Micro-Optics Elements, Systems and Applications, Taylor & Francis Ltd, Bristol, Pa., 1997, pp. 127-152. See also Sinzinger, Stefan, and Jahns, Jurgen, Microoptics, Weinheim, N.Y., 1999, pp. 85-123.
Finally, in the photothermal expansion technique certain materials (glasses, polymers), experience a local volume change when exposed to certain kinds of high intensity radiation. UV x-ray, electron, and proton beams have all been used to induce local volume changes in various materials, with the result that the exposed material is “squeezed” into a quasi-spherical lens shape.
Photothermal expansion techniques require particular photosensitive materials, and a high-energy radiation source, both of which impose cost and materials constraints. There also are limitations on the shape that the lenses can assume, and a post-exposure polish is often needed to ensure optical quality of the lenses. See for example, Kufner, Maria and Stefan, Micro-optics and Lithography, VUBPRESSS, Brussels, 1997., pp 81-118, 183-184. See also Herzig, Hans Peter, Micro-Optics Elements, Systems and Applications, Taylor & Francis Ltd, Bristol, Pa., 1997, pp127-152. See also Sinzinger, Stefan, and Jahns, Jurgen, Microoptics, Weinheim, New York, 1999, pp. 85-123.
2.1.2 Fabrication of Arrays of Microlenses by Hydrophobic Processes
Several authors have proposed and demonstrated techniques which enable the fabrication of microlens arrays by use of the hydrophobic effect. See M. E. Motamedi, et al., supra; P. Nussbaum, et al., supra; and also E. Kim and G. M. Whitesides, “Use of Minimal Free Energy and Self-Assembly To Form Shapes”, Chem. Mater. 7, 1257-1264 (1995).
The present invention will be seen to be a modification of these previous techniques in that, inter alia, microlenses will be assembled by use of hydrophilic domains patterned in an adhesive (rather than a chemically-bonded) hydrophobic layer. Polymer microlenses in accordance with the present invention will be seen to be readily fabricated on a useful variety of substrates, including glass (SiO2), Si, SiN, GaAs, InGaAs, and InP. See D. M. Hartmann, O. Kibar, S. C. Esener, “Characterization of a Polymer Microlens Fabricated Using the Hydrophobic Effect”, Optics Letters 25, 975-977 (2000).
The present invention will also be seen to be different from the suggested processes of these papers because, inter alia, it has been recognized that the focal length of the microlenses can be controlled by adjusting any of a number of parameters during the fabrication process. These parameters include the substrate tilt angle and withdrawal speed (if the lenses are formed by a process of withdrawing the substrate from a liquid bath), the liquid viscosity, surface tension, density, and index of refraction, the fill factor of lens arrays, and the surface-free-energy of the hydrophilic and hydrophobic regions of the substrate.
The present invention will still further be seen to be different from the suggested processes of these papers because it allows strong (low f-number) lenses to be fabricated via multiple dip-coats, or multiple rounds of condensation.
The present invention will yet still further be seen to be different from the suggested processes of these papers because, inter alia, it may employ a unique self-alignment strategy, as immediately next discussed.
2.2 Aligning Microlenses to Optical Components, and Direct Fabrication of Microlenses on Optical Components
There is a further present requirement to position microlenses in alignment to optical components. In particular, as optical systems have become more widespread increasing interest has arisen in the placement, or fabrication, of microlenses on or in precise spatial relationship to optically active devices such as vertical cavity surface emitting lasers (VCSELs), light emitting diodes (LEDs), and detectors, as well as directly upon passive components such as optical fibers. Such microlenses are used in coupling or guiding light from one optical component to another within an optical system.
Techniques for fabricating microlenses on or in fixed spatial relationship to other optical components naturally fall into two different categories: those in which the microlens is fabricated directly on the optical component, and those in which the microlens is fabricated externally, and is then aligned to the component of interest. Direct fabrication of microlenses has the advantage that the optical components themselves often define the footprints of the fabricated microlenses. For example, resist-reflow, ink-jet printing, and deep-proton irradiation have all been used to fabricate microlenses directly over the apertures of LEDs, VCSELs, and optical fibers using the apertures to define the microlens diameters. See, for example, P. Heremans, J. Genoe, and M. Kuijk, IEEE Photonics Tech. Letters 9, 1367 (1997); E. Park, et al., IEEE Photonics Technology Letters 11, 439 (1999); and M. Kufner, Microsystem Technologies 2, 114 (1996).
Similarly, microlenses have been fabricated on the ends of optical fibers by wet etching, laser ablating, or melting the ends of the fibers to produce microlenses. See, for example, Johnson et al., Proceedings of the SPIE 3740, 432 (1999); and Presby et al., Applied Optics 29, 2692 (1990).
Such microlenses can greatly improve the coupling efficiency of transmitters and detectors to optical fibers, and can even enhance the quantum efficiency of light-emitting devices. See, for example, P. Heremans, et al., supra; and also E. Park, et al., supra.
Direct fabrication of microlenses is useful when small optical beam diameters are acceptable. However free-space optical communication systems, optical switching systems, and many display and imaging applications, require relatively large beam diameters so that the beams do not appreciably diffract as they propagate through the system. In such cases, microlenses must be fabricated externally, some distance away from the output apertures of the optical components. This in turn requires careful alignment of the microlenses. Methods of performing such alignments include the use of precision grooves. See, for example, M. Rode and B. Hillerich, IEEE J. of Microelectromechanical Systems 8, 58 (1999).
Methods of performing such alignments also include micro-optical benches. See, for example, Y. Aoki et al., Applied Optics 38, 963 (1999); and also Y. Peter, Proc. of the SPIE 3513, 202 (1998).
Finally, methods of performing such alignments also include active-alignment using four-f imaging systems. The former schemes are somewhat limited in that they require the optical components to be placed into pre-fabricated structures, that have themselves been carefully aligned. The later active scheme, while more versatile, requires expensive machinery and can be extremely time consuming. When low f# microlenses are desired, alignment tolerances become so tight that the fabrication of microlenses aligned to optical components via any of these techniques becomes economically impractical. This limits the f# of microlenses that can be integrated and imposes minimum size constraints on the optical systems in which the microlenses are incorporated.
The first part of the present invention will be seen to show a new method for the low cost precision fabrication of microlenses. The second part of the present invention will show how, in an adaptation and extension of the base process, microlenses may be fabricated on, and in precision spatial relationship to, optical components.
2.3 Fabrication of Aligned Bump-bonds
Bump bonds, such as are commonly made with metal solders or with conductive polymer, must be aligned to the metal contact pads upon a substrate that commonly also contains electrical circuitry. The present invention will be seen to show how bump bonds may be fabricated in self-alignment to metal contact pads.
Solder bump bonding is by far and away the most widely used means of performing flip chip bonding. In the standard technique for placing solder bumps, a layer of solder (10 um thick or more) is thermally evaporated onto the chip, and lithographically patterned on top of wettable “base-metal” bonding pads. These bonding pads are surrounded by a non-wettable dielectric passivation layer, known as a solder dam. After lithographically patterning the solder, the substrate is heated to reflow the solder so that it forms a spherical bump, whose footprint is defined by the wettable base-metal pad area. A less-standard, but still widely used method of depositing the solder bumps is to electroplate them onto the base-metal bonding pads. There has even been some work conducted in ink-jet printing solder bumps on a circuit.
Polymer bump-bonds have also been explored for use in circuits, and have several important advantages over more conventional solder-bump bonds. These include a small size and weight, a reduction of processing costs, low-temperature curing capabilities, and the ability to easily rework flip-chip devices bumped with conductive adhesives.
Currently, there are several existing methods for fabricating polymer bump bonds. These include the use of stencils, screen-prints, and micromachining, to transfer a pattern of conductive polymer paste onto a substrate. Stencils and screens require careful alignment with the underlying bonding pads on the chip. The paste is then injected through the screen onto the bonding pads. Micromachined polymer molds have also been used to generate arrays of polymer bump bonds. The resulting polymer bonds must then be aligned to their bonding pads.
Like solder bump bonding, the present invention utilizes a wettable base-metal, surrounded by a non-wettable passivation layer to define the footprint of the polymer bumps. Uniquely, however, the liquid is self-assembled on the wettable base-metal, and there is therefore no need for any lithographic steps, reflow, or electroplating.
2.4 Fabrication of Bonds Between Perpendicular Substrates, and Other Three-Dimensional Microstructures
It is difficult to fabricated electrical bonds between very small features at the edge regions of perpendicular substrates because it is difficult to hold the substrates in precise alignment while the features are connected, such as by soldering of corresponding solder pads upon two perpendicular substrates.
Still other three-dimensional microstructures, such as optical fibers or light pipes, might usefully serve to connect corresponding regions upon different substrates that occupy different physical spaces, including perpendicularly proximate to one another.
The present invention will be seen to show the generation of connections and features, both electrical and optical, both (i) in situ, and (ii) self-aligned, between corresponding small, micro, domains located on different physical bodies, including substrates and boards including as may be in a perpendicular relationship.
2.5 Summary Attributes of the Prior Art
Accordingly, it is known to assemble, at least, microlenses by placement of liquids onto hydrophilic domains within a hydrophobic background; no particular method of liquid transfer being, however, described. It is in particular known to deposit liquids on hydrophobic areas to form microlenses. See, for example, Use of Minimal Free Energy and Self-Assembly to Form Shapes, Enoch Kim and George M. Whitesides, Chem. Mater., 1995, 7, Pgs. 1257-1264. See also Microcontact Printing of Self-Assembled Monolayers: Applications in Microfabrication, James L. Wilbur, Amit Kumar, Hans A. Biebuyck, Enoch Kim, and George M. Whitesides, Nanotechnology 7m 1996, Pgs. 452-457.
It is also known to cure these liquids to form stable polymer structures by ultraviolet (UV) light curing, by thermal curing, and by other means. See, for example, Kim and Whitesides, id.
The formation of microlenses by (i) pulling a substrate through a polymer/H2O interface has in particular been described. See, for example, Self-Organization of Organic Liquids on Patterned Self-Assembled Monolayers of Alkanethiolates on Gold, Hans A. Biebuyck and George M. Whitesides, Langmuir 1994, 10, Pgs. 2790-2793. See also Kim and Whitesides, id.
Microlenses have also been formed by (ii) putting a drop of polymer on the substrate and then tilting the substrate. See, for example, Combining Patterned Self-Assembled Monolayers of Alkanethiolates on Gold with Anisotropic Etching of Silicon to Generate Controlled Surface Morphologies, Enoch Kim, Amit Kumar, and George M. Whitesides, J. Electrochem. Soc., Vol. 142, No. 2, February, 1995 Pgs. 629-633
Microlenses have still further been formed by (iii) first condensing water on the substrate and then depositing the polymer subsequently, so that the polymer goes only to the non-hydrophilic areas of the substrate. See, for example, Thin Microstructured Polymer Films by Surface-Directed film formation, H.-G. Braun, E. Meyer, appearing in Thin Solid Films 345 (1999) Pgs. 222-228.
The present invention will be seen to use a process step other than (i) passing through a polymer/H2O interface; or (ii) tilting; or (iii) condensing water/depositing polymer.
Quite logically, the prior art recognizes the use of microlens arrays to focus and correct aberrations in the intensity of light from lasers or optical fibers. See, for example, Biebuyck and Whitesides, id. However, the location and alignment of the arrayed microlenses relative to the light sources—as will be taught by the present invention—is problematic.
Finally, it is also known in the prior art to use conductive polymers for electrical connection, and to assemble the conductive materials (the conductive polymers) by dip-coating on hydrophilic domains. See, for example, Selective Deposition of Films of Polypyrrole, Polyaniline and Nickel on Hydrophobic/Hydrophilic Patterned Surfaces and Applications, Z. Huang, P. C. Wang, J. Feng, and A. G. MadDiarmid, Synthetic Metals 85 (1997) Pgs. 1375-1376.
However, to the best knowledge of the inventors, the prior art deals exclusively with the use of conductive polymer to make electrical contacts (that is, pads to which bump bonds may be attached). The present invention will shortly be seen to contemplate another use: the making of the bump bonds themselves.